Attempts have been made to provide sorters for use by individual carriers, such sorters having the envelopes or documents handled thereby stacked in a direction perpendicular to the face of the envelopes.
Because of this type of perpendicular stacking, stacked groups of sorted envelopes or documents assume the shape of an irregular bundle, and each of the several bins or stackers employed contains one of these bundles of envelopes or documents (hereinafter called "letters"). Because the letters depend on one another for support, handling and manipulating the bundles must be done very carefully. The bundle configuration is aggravated by the inherent random mix of letter sizes. If a bundle is to be transferred from one location to another, (for reprocessinq the letters for the second pass, for example, or for transferring them into mail trays), each bundle must be supported so that letters don't "squeeze" out of the center of the bundle, which would cause the bundle to collapse with loss of sequence and facing. Another problem with handling discrete bundles of letters is the re-assembly of the individual bundles into one continuous bundle or stack for re-processing on the second pass.
Additionally, present sorters stack letters with one edge of each letter moved against a reference edge. When letters are to be processed for the second pass sortation, they must be resingulated. Even though each letter was singulated on the previous pass, when it proceeded before the bar code or character reader, none of that singulation is retained because of the edge registration.
Further, because of the difficulty in manipulating bundles of letters, sequenced letters cannot be automatically placed into mail trays without employing complicated and expensive robotic techniques. Such robotic techniques generally require a high level of skill for maintenance and, hence, have a continuing high cost of operation factor. Because of the aforementioned problems, present sorting machines depend on manual removal of sequenced letters from the bins or sorters at the conclusion of sorting. This manual removal of mail from stackers, bins or sorters is commonly referred to as "sweeping".